Microglia are critical the primary complications of the human immunodeficiency virus (HIV-1) in the central nervous system (CNS), since they are the most commonly infected cell and their infection represents the majority of the viral load in the CNS. This proposal will center on the biology of HIV-1 in microglia in order to develop a better understanding of the role of the virus in the development of this complication, and to help identify potential, CNS-specific treatment strategies. In the first specific aim we will continue our studies on microglial- tropism of HIV-1 isolates using primary isolates from adults and children, and an isolated adapted to microglial by sequential passage. We will first use a PCR-based assay to analyze the sequential steps of HIV-1 infection in microglial. For those isolates (like the PCR-based assay to analyze the sequential steps of HIV-1 infection in microglia. For those isolates (like the microglia-adapted HIV-1/BOR-15) which demonstrate a rapid entry phenotype, we will molecularly clone the envelopes, and define the mechanism of enhanced cellular penetration suing molecular and biochemical (binding) assays. Where tropism for microglial cells is related to post-entry steps, other portions of the pro-virus, or the entire proviral genome, will be cloned. We will then determine whether isolates that do not replicate to high levels in microglia can nevertheless establish a chronic infection. In the second specific aim we will determine whether the envelope proteins, and specifically gp120 from isolates with HIV encephalopathy can mediate changes in intracellular free Ca2+ concentrations in monocyte-derived macrophages (MDM), microglia, and neural cells. Those gp120s that induce intracellular signals will be tested for their ability to mediate apoptosis. In the third specific aim we will determine whether microglial infection by certain HIV isolates results in increased production of chemokines, which could be responsible for increased cellular trafficking into the CNS, and potential amplification of a chronic infection. The results from these experiments will strengthen knowledge about the interactions between HIV-1 and microglial cells.